Britain's reaction to the invasion of French Indochina in 1941?

Agreed, the OP question was not what the British should have done, but what they did, hence my reply.

Fair enough

What the British did was increase the size of the Malaya Garrison from a Brigade to 7 Brigades in 3 divisions (The very green and under equipped 8th and 11th Indian and the green 8th Australian) and split the 3rd Australian Brigade of the 8th AID among 3 'barrier' sites (Ambon, Timor and Rabaul).

They also sent the 18 British Infantry Division to Singapore where it arrived piecemeal and without much of its equipment after the fighting started.

The RAF sent its older or less useful aircraft - including all but one of the porcine Brewster Buffalo fighter planes - to increase the Garrisons air support - but this was still less than half of what would have been required and the air defence network was virtually non-existent.

Lastly the RN sent 2 Capital ships and was sending a carrier to act as a deterrence as force Z.

In Hong Kong they were initially going to draw down forces but quite late on a decision was made to instead reinforce the place and Canada sent a Pair of Infantry battalions with a Brigade HQ and support units which arrived in late 41 - although a lot of its equipment did not arrive and ended up being used elsewhere.

All in all it was not enough (or in the case of HK too much as the place could never be held and those troops would have been better used in Malaya or in one of the barrier locations).

But as said its hard to see what else could be sent

I guess with hindsight Divisions in the UK (of which about 10 fully trained formations existed out of the 28 odd in the country at the time) could have been freed up and sent to Africa (North and East) allowing some or all of the Indian and Australian divisions to go to the Far East.

Maybe Canadian units could be sent as well if the Canadian government agreed?

The same with air units - RAF fighter command giving up its hold on several Spitfire fighter wings earlier to allow them to go to North Africa and replace the P40 Squadrons and for those to head to the Far East as well as the RAAF and RNZAF Buffelo squadrons re-equipping with the P40s

Those units would have presented a far harder challenge to the Japanese units both on the ground and in the air and would have provided a greater leadership pool for the commonwealth in the region.

The RN is hard pressed to do much else - only a POD such as a more comprehensive defeat of the RM earlier (Op Judgement with bells on) with fewer subsequent losses might serve to free up forces for a larger Force Z????
 
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